Polyolefin materials, such as homopolymers and copolymers of propylene, which are modified so as to be polar, have many uses. Polar-functionalized polypropylene materials, for example, can be used as additives for or in plastic products to modify the surface characteristics or preparation processability of such products. Functionalized propylene homopolymers and copolymers can also be used for the preparation of aqueous dispersions for car and cleaning products, and may be especially useful as additives for hot melt adhesive formulations to modify the adhesive properties thereof.
Frequently, acid functionalization of propylene homopolymers and copolymers by grafting acidic moieties involves reaction of the molten propylene-based polymer with an unsaturated carboxylic acid or derivative such as maleic anhydride in the presence of a free radical initiator such as an organic peroxide. Preparation of functionalized propylene-based polymers in this general manner is disclosed, for example, in U.S. Pat. Nos. 4,753,997; 5,001,197; 5,319,030; 5,728,776; 5,998,547; 6,228,948; 6,331,595; 6,437,049 and 6,784,251; and in U.S. Patent Applications 2002/0026010 and 2004/0054086.
The manner in which, and the extent to which, acid functionalization, e.g., maleation, of propylene-based polymers (hereinafter called “polypropylene” whether they are homo- or co-polymers) occurs can affect the properties, characteristics, and usefulness of the resulting functionalized polymer-containing reaction product. If too little of the functionalizing acid reactant is grafted to the polymer backbone compared to the total amount of functionalizing agent used, the residual unreacted acid reactant within the reaction product can cause problems.
For example, ungrafted maleic anhydride in a maleation operation, especially if present under reaction conditions such as relatively high temperatures, can result in side products being formed during and after the maleation reaction. When the resulting reaction product is then combined with non-functionalized polyolefin material, for example, in a hot melt adhesive formulation, these side products can cause undesirable color to be imparted to the adhesive formulation. Furthermore, these residual unreacted impurities can lead to poor color stability, polymer molecular weight degradation and char formation upon exposure of the adhesive product to heat. Finally, a hot melt adhesive using a maleation reaction product containing too much of free maleic anhydride or the maleic anhydride-based side products may cause undesirable haze to form within the adhesive product which is preferably clear and haze-free.
The foregoing problems which can be encountered with commercial scale production of functionalized polypropylene-containing reaction products may be eliminated or minimized by improving the efficiency of the grafting (to a given desired extent) of functional moieties onto the polypropylene backbone. It would therefore be advantageous to identify selected combinations of reactants and reaction conditions which can be utilized to realize suitable Grafting Efficiency in preparation of materials of this type which have been functionalized to a certain Grafting Content.
Grafting Efficiency is a quantitative measure of the concentration of carboxylic acid-based functionalizing agent which is covalently bound to the polypropylene backbone. It is defined as the concentration of functionalizing agent grafted onto the polymer backbone as a percentage of the concentration of functionalizing agent within the crude reaction product. Grafting Content quantifies the total amount of functionalizing acid groups which have been grafted onto the polymer backbone in terms of acid number, i.e., the milligrams of neutralizing KOH needed per gram of functionalized polymer.
It may be advantageous to provide effective and suitable values for Grafting Efficiency and Grafting Content, using reaction conditions which do not themselves create or impart undesirable characteristics (e.g., poor color or reduced molten clarity) within the desired polymeric reaction product or within the eventual end products to which such functionalized reaction products may be added.